A fascinating study here about rapid fertility decline occurring in homogeneous populations. The most homogeneous populations in the world are experiencing declining birth rates. I've only read the first page, shown here, but the theory is amazing. It is thought that the idea of contraception - and the modern notion of parenthood being burdensome - has spread through these populations uniformly. Ideologically, there is less variation among individuals in communities where people share the same religion, cultural practices, etc. So, the thinking is that, ultimately, people practice contraception even when they don't need to.

This is a little simplistic, but on the whole it seems valid. Ethnic Italians have one of the lowest birthrates in the world - way below replacement levels - but most couples still have one child, to appease social norms.

This reminds me of another study I heard about recently, that the most fecund potential mate is in fact not someone totally genetically dissimilar to you, but a distant relation such as a 3rd cousin. The inference is that since most people in industrialised countries have an enormous pool of potential mates, the odds of pairing with a distant relative - and having a dozen kids - is radically reduced, and so with that falls fertility.

This is interesting. I also remember reading something on the same (sort of) subject a while back. I think it was about a study which found that male sperm production has declined by almost half over the past century due to several factors which I can't remember right now. But that's actually quite far from what this study is about.